The present invention is related to a process for the manufacturing of non-reinforced load carriers of thermoplastic material.
Thermoplastic products are used in many different areas. These products can be achieved through a number of different manufacturing methods, the most commonly present being injection moulding, vacuum moulding, blow moulding and press moulding.
There is a desire to achieve high load absorption capacity within certain areas. Carrying structures made of materials such as concrete and steel will be able to withstand substantially the same load, independent of temperature and time. This is not the case with thermoplastic materials where a relatively light load might cause a remaining deformation at extended exposure. This phenomena is called creep Strain or cold flow. A structure made of thermoplastic material will however be able to cope with loads that are tens of times higher at shorter times, without any remaining deformation The amount of cold flow in respect of time and temperature is depending on thermoplastic material type and quality.
Load carrying thermoplastic products will most often have to be designed for the most unfavourable load i.e. longest time and highest temperature it might be exposed to during its useful life. It is, however, possible to reduce the amount of cold flow by adding fillers or reinforcing fibres to the thermoplastic material. Common fillers are usually minerals such as lime or mica while reinforcing materials usually comprises glass fibre, steel fibre or carbon fibre. It is also possible to reinforce a thermoplastic product by integrating a metal design in the product. This may for example be constituted of a metal rod placed in a hollow chamber intended for the purpose. Such additives will, however, deteriorate other desired properties the thermoplastic material, by nature, is provided with. One such property is that most thermoplastic materials are easy to keep clean and hygienic. This property will be lost when fillers are added. It will furthermore be impossible, or at least very difficult, to recover the material, both in cases where reinforcing bars and where fillers are used. Recycling of thermoplastic materials are almost mandatory nowadays.
It has, through the present invention, quite unexpectedly, been made possible to achieve non-reinforced load carriers of thermoplastic material with a very high resistance towards cold flow. Non-reinforced means that the load carriers are free from reinforcing additives such as various forms of fibres added to the thermoplastic material and also free from reinforcing profiles, beams or bars made of a material not compatible with the material of the load carrier. Other reinforcements such as material elevations or profiles made of the same material as the load carrier itself may of course occur. The invention relates to a process for the manufacturing of non-reinforced load carriers of thermoplastic material. The process comprises the steps extrusion, vacuum moulding and/or blow moulding of a thermoplastic material such as polyethylene, polypropylene or polybutene. The invention is characterised in that;
a, i) Work pieces in the shape of sheets are manufactured by means of an extruder. The work pieces are allowed to cool and post shrink uniformly c after the manufacturing, suitably by an intermediate storing of 6-48 hours.
a, ii) Extended bar sections with a preferably rectangular or polygonal cross-section are also manufactured by means of an extruder The bar sections are allowed to cool and post shrink uniformly after the manufacturing, suitably by an intermediate storing of 6-48 hours.
b, i) An upper deck member is then manufactured by heating and then moulding two of the uniformly post shrinked work pieces in a first mould comprising a first and a second mould half. The mould halves comprises one shaping cavity each. The two shaping cavities are together, a negative representation of an upper deck member to a load carrier. The mould halves are arranged so that an intermediate space is formed between them and so that the cavities are facing each other. The two heated work pieces are hereby placed in the intermediate space between the two cavities after which they are moulded in each one cavity under influence of vacuum and/or pressure. The mould halves are then pressed together while the thermoplastic material is still hot so that the two work pieces are welded with each other and forms a hollow unit. The thermoplastic material is then allowed to cool somewhat before the mould is opened, the upper deck member is removed and the process can be repeated.
b, ii) A lower deck member is also manufactured by heating and then moulding two further, also uniformly post-shrinked, work pieces which after heating is formed in a second mould comprising a first and a second mould half The mould halves each comprises one shaping cavity, which cavities together is a negative representation of a lower deck member to a load carrier. The two heated work pieces are hereby placed, moulded, welded and removed in a manner similar to the manufacturing of the upper deck member.
b, iii) A skid is possibly also manufactured by heating and then moulding two of the extended bar sections in a third mould comprising a first and a second mould half comprising a first and a second mould half, which mould halves each comprises one shaping cavity, which two cavities together is a negative representation of a skid to a load carrier. The two heated bar sections are hereby placed, moulded, welded and removed in a manner similar to the manufacturing of the deck members.
c) The upper and the lower deck member and the possible skid are then allowed to cool and post shrink uniformly, after being removed from the mould, suitably by an intermediate storing of 6-24 hours.
d) A number of joining surfaces on the lower side of upper deck member, on the upper and lower side of the lower deck member and on the upper side of two or three bar sections or alternatively two or three skids, are then joined with each other. The surfaces are, in one or more steps, heated by means of infrared heating, laser or by being pressed against a heated weld core. The different parts are, in one or more steps, pressed together so that the molten thermoplastic material in the lower joining surfaces of the upper deck member and on the upper and lower joining surfaces of the lower deck member as well as the joining surfaces of the bar sections or the skids weld or melt joins. The thermoplastic material in the joining surfaces is then allowed to cool and solidify at least partly before the pressing is discontinued. A joined, non-reinforced load carrier of thermoplastic material is hereby achieved.
The sheet shaped work pieces are suitably oriented so that the direction of the extrusion coincides between the layers that forms the upper and lower deck members. The reason to this is that the contraction of the thermoplastic material is depending on the direction of the polymer chains These polymer chains will be oriented during the extrusion process. Even if the main part of the contraction will take place within a couple of days, some additional contraction will occur during the main part of the useful life of the product. This contraction will be accelerated when the product is exposed to increased temperatures, such as for example during washing and drying. This contraction might cause bimetal-like warping of the product in cases where the layers aren""t oriented in the same direction. Such a warping will most certainly increase as the product ages.
The post-shrinking of different parts is of the same reason controlled so that all parts included in one and the same load carrier is in mainly the same degree of contraction This can be achieved in different ways One way is to allow the work pieces to shrink so that at least 75% of the total contraction is obtained. This can be achieved by an intermediate storing of the parts from a couple of hours to a couple of days before the next step in the process is started. It is here important that the different parts has the same degree of contraction.
The contraction can be accelerated by letting the intermediate storing take place in an elevated temperature, for example 60-100xc2x0 C. The later will also give the advantage that the heating will be less time consuming when for example moulding the deck members. An accelerated contraction might, however, be less suited before the final assembly as this will require some stability in the different parts. Another way to solve the contraction problem is to xe2x80x9cfreezexe2x80x9d the different parts directly after each step of the process which involves thermoforming. This is suitably achieved by rapidly cooling the parts to below room temperature immediately after the moulding. The contraction will then stop almost completely and will not commence until the parts are heated.
It is also possible to mould and assemble all parts at predetermined junctures in the beginning of the contraction, for example when only 5-10% of the total contraction is obtained. The moulding of the deck members can for example take place 1 minute xc2x15 seconds and the welding 5 minutes xc2x110 seconds, after the extrusion of the work pieces.
The bar sections, which in the finished load carrier constitutes skids, are suitably massive. These bar sections are milled to the desired dimensions after cooling and post-shrinking, but before the assembly, whereby undesired cross-warping in the skid can be avoided.
These skids are alternatively hollow, extruded, profiles which preferably after cooling and post-shrinking and possibly before assembly is sealed at its both ends by means of plugs or by being thermoformed. The skid may according to yet another alternative be manufactured in the same manner as the deck members whereby it comprises an upper and a lower layer The upper layer is preferably connected with the lower layer via distance sections The distance sections forms an integrated part of either or both of the layers and are moulded together with the moulding of the respective layers.
The thermoplastic material used for manufacturing is preferably a polymer, for example polyethylene, polypropylene or polybutene with an average molecular weight in the range 200,000-2,000,000 preferably greater than 300,000. It is, in certain cases, as for example when moulding parts with thinner material thickness advantageous to use a material with the molecular weight in the range 1,000,00-2,000,000, while thicker parts most often are produced of a material with a molecular weight in the range 300,000-1,000,000.
The deck members are preferably provided with mainly vertical surfaces and mainly horizontal surfaces, whereby the joining surfaces are arranged on horizontal surfaces. Vertical surfaces are further arranged adjacent to, or envelopes said joining surfaces. The joining surfaces suitably has a goods thickness which is greater than the goods thickness of the sections located adjacent to the joining surfaces.
The upper deck member and the lower deck member are each constituted of an upper layer and a lower layer, which upper layers preferably are connected with its respective lower layer via distance sections. These distance sections constitutes an integrated part of the respective layer and are moulded in connection to the moulding of the respective layers. It is suitable to join the two surfaces when larger flat surfaces are present by joining them with one another by means of tower- or ridge-like elevations This will increase the rigidity and at the same time improve the dimension stability.
The different parts are preferably joined with each other by means of welding such as mirror welding, laser welding, friction welding and or filler rod welding.
Mirror welding is performed by heating two surfaces of thermoplastic material until it melts. The heating is performed by means of a so called weld core which is a plate made of metal. The heated thermoplastic surfaces are then pressed together while the molten material is allowed to cool.
When laser welding, one of the layers is at least somewhat translucent while the other one is opaque, most often by adding carbon black to the thermoplastic material. The two layers are pressed together whereby the surfaces to be joined are illuminated with a laser. The illumination is performed from the translucent side. The energy from the laser beam will be transformed into thermal energy when it hits the opaque layer whereby it melts and the parts are joined by welding.
When friction welding, the surfaces to be joined are rubbed against each other so that the material melts due to the friction heat. Commonly known friction welding methods are ultrasonic welding, low frequency welding and rotation welding.
Welding with filler rod in thermoplastic materials is similar to its metal counterpart The joining surface and the filler rod, which are made of the same thermoplastic material are heated with a so-called hot air insulator. The filler rod is pressed into the joint while the hot air beam is slowly moved along the joint This method may also be used as a complement in combination with the ones mentioned above.
Predetermined surfaces on the load carrier is suitably coated with friction enhancing material. The surfaces to be coated are suitably pre-treated by flame or corona treating whereupon the surface is spray coated with the friction enhancing material. The friction enhancing material comprises, for example, ethyl-vinyl-acetate or the combination polyurea-prepolyrner and diisocynate. The friction coating is suitably performed after the joining of the different parts of the load carrier. The parts partly or completely coated are;
the upper side of the upper deck member,
the lower parts of the bar sections, and
selected parts of the lower side of the lower deck member.
The process according to the present invention is suited for the manufacturing of pallets where the demands for mechanical stability are very high and where reinforcing profiles or additives are banished due to hygienic or environmental reasons. The process may also be used for other types or load carriers such as foldable or solid pallet containers and bulk containers.
The invention is illustrated further through a process scheme and through an enclosed FIGURE which shows one embodiment of the invention whereby,
The figure shows a process for the manufacturing of an embodiment of a load carrier 1 with an upper deck member 11, a lower deck member 12 and a skid 3xe2x80x2. 